1. Field of the Invention
The present invention relates to a system and assembly for limiting the egress of particles from a material size reduction process based on the particle size, and more particularly, it concerns an improved classifier assembly for a rotary coal pulverizer.
2. Background of the Related Art
In operations that use coal for fuel, finely-ground coal particles or “fines” are required for efficient operation. The pulverized firing of lower-cost lower-grade coal, coke, or lignite yields higher combustion efficiency than stoker firing, as well as rapid response to load changes. Thus, it is common practice to supply raw coal to a device that will reduce the size of the coal to particles within a desirable range, such as a pulverizer, prior to being used for combustion.
Many pulverizers employ systems and methods including one or more crushing and grinding stages for breaking up the raw coal. It is important to maintain close control over the size of the pulverized coal used for combustion because finely pulverized coal produces less nitrous oxide (NOx) emissions and keeps oversized loss-on-ignition (LOI) unburned coal particles from contaminating the marketable ash byproduct of the combustion chamber, among other things. Coal particles leaving the device have to be reduced to dust fine enough to become airborne by repeated crushing actions of the rolling or flailing elements of the device. Air is swept through the device and the dust particles are entrained in an air stream and carried out for combustion.
There is a need for the classification of the solid coal particles on the basis of particle size so that only acceptably sized fines exit the pulverizer, while the egress of unacceptably sized particles is limited, and the particles remain for further size reduction. Some pulverizers require an add-on device, such as an external classifier, to separate and recycle unacceptably large sized particles back to the pulverizer. Other coal pulverizers, such as rotary coal pulverizers, include internal rejector assemblies that operate along with the pulverizer to block unacceptably large sized particles so that these particles stay within the pulverizer for further size reduction, while allowing acceptably sized particles to move through the rejectors. In these rejector assemblies, it is often difficult to install and maintain the rejector arm gap at the desired size. Furthermore, wear and tear on the assembly can further increase the difficulty of maintaining a consistent rejector arm gap.
However, no prior art coal pulverizer includes an internal system that is as uniquely effective at limiting the egress of unacceptably sized particles as the present invention disclosed herein below. Rotary coal pulverizers in particular can advantageously provide an exceptionally fast processing time, less transport time to the burners, and an easily controllable input/output flow relationship, among other benefits.